The Stratospheric Observatory for Infrared Astronomy (SOFIA) is an airborne observatory for astronomical observations at wavelengths ranging from 0.3-1600 µm. It consists of a telescope with an effective aperture of 2.5 m, which is mounted in a heavily modified Boeing 747SP. The aircraft features an open port cavity that gives the telescope an unobstructed view of the sky. Hence the optical system is subject to both aerodynamic loads from airflow entering the cavity, and to inertial loads introduced by motion of the airborne platform. A complex suspension assembly was designed to stabilize the telescope. Detailed end-to-end simulations were performed to estimate image stability based on the mechatronic design, the expected loads, and optical influence parameters. In December 2010 SOFIA entered its operational phase with a series of Early Science flights, which have relaxed image quality requirements compared to the full operations capability. At the same time, those flights are used to characterize image quality and image stability in order to validate models and to optimize systems. Optimization of systems is not based on analytical models, but on models derived from system identification measurements that are performed on the actual hardware both under controlled conditions and operational conditions. This paper discusses recent results from system identification measurements, improvements to image stability, and plans for the further enhancement of the system.
During observation flights the telescope structure of the Stratospheric Observatory for Infrared Astronomy (SOFIA) is
subject to disturbance excitations over a wide frequency band. The sources can be separated into two groups: inertial
excitation caused by vibration of the airborne platform, and aerodynamic excitation that acts on the telescope assembly
(TA) through an open port cavity. These disturbance sources constitute a major difference of SOFIA to other ground
based and space observatories and achieving the required pointing accuracy of 1 arcsecond cumulative rms or better
below 70 Hz in this environment is driving the design of the TA pointing and control system. In the current design it
consists of two parts, the rigid body attitude control system and a feed forward based compensator of flexible TA
deformation. This paper discusses the characterization and control system tuning of the as-built system. It is a process
that integrates the study of the structural dynamic behavior of the TA, the resulting image motion in the focal plane, and
the design and implementation of active control systems. Ground tests, which are performed under controlled
experimental conditions, and in-flight characterization tests, both leading up to the early science performance capabilities
of the observatory, are addressed.
SOFIA, the Stratospheric Observatory for Infrared Astronomy, is an airborne observatory that will study the universe in
the infrared spectrum. A Boeing 747-SP aircraft will carry a 2.5 m telescope designed to make sensitive infrared
measurements of a wide range of astronomical objects. In 2008, SOFIA's primary mirror was demounted and coated for
the first time. After reintegration into the telescope assembly in the aircraft, the alignment of the telescope optics was
repeated and successive functional and performance testing of the fully integrated telescope assembly was completed on
the ground. The High-speed Imaging Photometer for Occultations (HIPO) was used as a test instrument for aligning the
optics and calibrating and tuning the telescope's pointing and control system in preparation for the first science
observations in flight. In this paper, we describe the mirror coating process, the subsequent telescope testing campaigns
and present the results.
The telescope pointing control of the Stratospheric Observatory for Infrared Astronomy (SOFIA) is achieved during
science observations by an array of sensors including three imagers, gyroscopes and accelerometers. In addition,
throughout alignment and calibration of the telescope assembly, the High-speed Imaging Photometer for Occultation
(HIPO) is used as a reference instrument. A summary of the telescope pointing control concept is given and how HIPO
is used to calibrate the telescope reference systems on the sky. A method is introduced using simple maneuvers to
perform initial alignment of HIPO, the imagers and the gyroscopes by means of single star observations. During the first
on sky testing of the SOFIA telescope, these maneuvers were carried out and the alignment could be improved
iteratively. The corresponding alignment accuracies are identified considering repeated measurements, environmental
and sensor noise. Inertial and non-inertial observations, as well as measurements over the entire operational elevation
range provide additional alignment and sensor performance information. Finally, an overview is presented for future
improvements in alignment.